Reactive polyurethane hotmelt adhesives in the context of the present invention are moisture-curing or moisture-crosslinking adhesives which are solid at room temperature, which are applied in the form of a melt and of which the polymeric constituents contain urethane groups and reactive isocyanate groups. Cooling of the melt results first in rapid physical setting of the hotmelt adhesive followed by a chemical reaction of the isocyanate groups still present with moisture from the environment to form a crosslinked infusible adhesive.
Lamination adhesives may either be similar in composition to reactive hotmelt adhesives or are applied as one-component systems from solutions in organic solvents, another embodiment consisting of two-component solvent-containing or solventless systems in which the polymeric constituents of one component contain urethane groups and reactive isocyanate groups while the second component contains polymers or oligomers bearing hydroxyl groups, amino groups, epoxy groups and/or carboxyl groups. In these two-component systems, the component containing isocyanate groups and the second component are mixed immediately before application, normally using a mixing and metering system.
Reactive polyurethane adhesive/sealing compound, more particularly one-component moisture-curing systems, generally contain polymers liquid at room temperature containing urethane groups and reactive isocyanate groups. For many applications, these compositions are solvent-free and extremely viscous and paste-like, being applied at room temperature or at a slightly elevated temperature of around 50.degree. C. to 80.degree. C.
Moisture-curing or moisture-crosslinking polyurethane primers are primers of which the polymeric constituents contain urethane groups and reactive isocyanate groups. In general, these compositions contain solvents and are applied at room temperature. To accelerate evaporation of the solvent, the substrates precoated with the primer may optionally be lightly heated.
Foam plastics from non-reusable pressurized containers in the context of the present invention are produced where they are to be used so that they are also known as in situ foam (DIN 18159). The systems in question are, in particular, moisture-curing one-component systems. The composition to be foamed is generally accommodated in non-reusable pressurized containers (aerosol cans). These polyurethane foams are used above all in the building field for sealing, insulation and assembly, for example of joints, roofs, windows and doors.
Reactive polyurethane compositions based on prepolymers of polyols and polyisocyanates and other additives are used in all these fields of application. In the case of the one-component formulations, the isocyanate groups still present react with moisture from the surrounding atmosphere, the reaction being accompanied by crosslinking and--in the case of the foams--an increase in volume. In the case of the two-component systems, chemical curing is accompanied by enlargement of the molecule or crosslinking of the molecule with the hydroxyl groups, amino groups, epoxy groups or carboxyl groups of the second reactive component.
The reactive polyurethane compositions for the applications mentioned above are distinguished by a very high performance profile. Accordingly, new applications for these adhesive compounds, sealants, foams and primers have been increasingly established over recent years. Compositions for such adhesives, sealing compounds, foams and primers are already known from a very large number of patent applications and other publications.
In addition to many advantages, the polyurethane compositions in question are also attended by certain disadvantages inherent in the system. One of the most serious of these is the residual monomer content of polyisocyanates, more particularly the more volatile diisocyanates. Adhesives, sealing compounds and, in particular, hotmelt adhesives are applied at elevated temperatures. Hotmelt adhesives are applied, for example, at 100.degree. to 170.degree. C. while lamination adhesives are applied at 70.degree. to 150.degree. C. At temperatures in these ranges, monomeric diisocyanates have a considerable vapor pressure. Primers are normally applied by spraying. The isocyanate vapors formed in all these methods of application are toxic on account of their irritant and sensitizing effect, so that precautionary measures have to be taken to prevent damage to the health of personnel involved in the application process. These measures, including for example the obligation to ensure that the maximum workplace concentration is not exceeded, extraction of the vapors at the point of formation and elimination, are highly cost-intensive and, in addition, are an obstacle to certain methods of application, for example spraying, of the reactive polyurethane compositions. The special problems posed by polyurethane foams from aerosol cans will be discussed hereinafter.
Accordingly, the development of reactive polyurethane compositions with a drastically reduced content of monomeric diisocyanates is highly desirable for all the applications mentioned and, in some cases, would actually enable them to be used for the very first time in many applications where their use has not hitherto been possible for the reasons explained above.
Several methods have hitherto been adopted to solve this problem:
According to the Schulz-Flory statistic, the content of monomeric diisocyanate in the reaction product where diisocyanates containing isocyanate groups of equal reactivity are used is dependent on the NCO:OH ratio used in the synthesis of the prepolymer: ##EQU1## where .beta. is the content of monomeric diisocyanate and a is the NCO:OH ratio (see, for example, R. Bonart, P. Demmer, GPC-Analyse des Gehaltes an monomerem Diisocyanat in Prepolymeren von segmentierten Polyurethanen (GPC Analysis of the Content of Monomeric Diisocyanate in Prepolymers of Segmented Polyurethanes, Colloid and Polymer Sci., 260 518-523 (1982)). With an NCO:OH ratio of 2, as frequently necessary for the composition of the prepolymer, 25% of the diisocyanate used remain as monomer in the prepolymer. If, for example, 10% by weight of diphenyl methane diisocyanate (MDI) are used for synthesis of the prepolymer at an NCO:OH ratio of 2, approximately 2% by weight of monomeric MDI is found in the prepolymer in accordance with the above statistical estimation. At an application temperature of 150.degree. C., the MDI already has a vapor pressure of around 0.8 mbar. Under the application conditions described above, particularly if the composition is applied in thin layers as an adhesive over large surface areas, considerable quantities of this residual monomer enter the overlying air space and have to be removed by extraction.
To reduce this monomer content by one power of ten, the NCO:OH ratio would have be drastically reduced to around 1.19. However, this is generally not practicable in practice because the average molecular weight would increase exponentially and the resulting polyurethane compositions would have extremely high viscosities and could no longer be applied. In practice, therefore, different methods are adopted for synthesis of the prepolymer. For example, the prepolymer is synthesized with a sufficiently high NCO:OH ratio and, after formation of the prepolymer, the monomeric diisocyanate or polyisocyanate is removed in a second step, for example by
distillation in vacuo (thin-layer evaporator), see for example Kunststoff-Handbuch, Vol. 7, Polyurethane, G. W. Becker (editor), Hanser Verlag Munchen, 3rd Edition, 1993, page 425, PA1 subsequent chemical fixing of the monomeric diisocyanate or polyisocyanate, for example by partial trimerization and working up of the reaction mixture, cf. for example the reference from loc. cit., page 97: K. C. Frisch, S. L. Reegen (editors): Advances in Urethane Science and Technology, Vol. 1-7, Technomic, Westport, Conn., 1971-1979. PA1 50 to 90 and preferably 60 to 85% by weight of isocyanate prepolymers, PA1 0.1 to 5.0 and preferably 0.5 to 2.0% by weight of catalysts, PA1 5 to 35 and preferably 10 to 25% by weight of blowing agents and PA1 0.1 to 5.0 and preferably 0.5 to 3.0% by weight of foam stabilizer.
These processes are applied, for example, in the production of low-viscosity adducts or solvent-containing adducts such as, for example, hardeners for paint binders. In the case of the highly viscous polyurethane adhesives, they have not acquired any significance on account of the high outlay on equipment they necessitate.
Accordingly, the problem addressed by the invention was to provide a reactive polyurethane composition in which the content of monomeric volatile isocyanates (essentially diisocyanates) would be so low that there would no longer be any need for expensive extraction or other safety measures during their practical application for reasons of safety in the workplace. In the case of the foam plastics, the disposal of residues of the prepolymers remaining in the non-reusable pressurized containers would not be problematical. The special problems involved are discussed hereinafter.